277v-120v transformer problem

Exactly what two points are you reading 28V between? Is that from ground to one of the secondary leads? If so, the problem is that the secondary is floating and not bonded to ground. Measure between the two secondary leads. You have to ground one of them.

277v-120v transformer problem

If you have two primary leads on the transformer and two secondary leads on the transformer, you do not need any grounding to see the output voltage. If you want additional feedback I think you need to give us a better description of the setup. What is the potential use of this transform? How big is it? How about a picture